Radiation-emitting semiconductor body for a vertically emitting laser and method for producing same

ABSTRACT

The present invention concerns a radiation-emitting semiconductor body with a vertical emission direction, a radiation-generating active layer, and a current-conducting layer having a current-blocking region and a current-permeable region, the semiconductor body being provided for a vertically emitting laser with an external resonator, and the external resonator having a defined resonator volume that overlaps with the current-permeable region.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional, and claims benefit under Section 120,of U.S. patent application Ser. No. 11/210,263, filed Aug. 23, 2005 nowU.S. Pat. No. 7,443,898, which claims priority to German Application No.10 2004 042 510.8, filed Aug. 31, 2004, and German Application No. 102005 036 820.4, filed Aug. 4, 2005. The contents of the priorapplications are incorporated herein by reference in their entirety.

BACKGROUND

The present invention relates to a radiation-emitting semiconductor bodywith a vertical emission direction.

A radiation-emitting semiconductor laser of this kind is known forexample from the document WO 02/13334 A2. Described therein is avertically emitting laser in the form of a so-called VCSEL (VerticalCavity Surface Emitting Laser). This VCSEL comprises a semiconductorbody with a laser resonator formed by two resonator mirrors and with,inter alia, an active layer and a current constricting layer disposed inthe laser resonator. The current constricting layer serves in operationto concentrate the operating current on a small subarea of the activelayer in order to create in that subarea the population inversionnecessary for laser operation.

Such surface-emitting VCSELs excel in terms of their high radiationquality, but have a comparatively low optical output power. In addition,as described in the above-cited document, the current conduction must bewell defined, since otherwise, due to the proximity of the current pathand the laser resonator volume, combined with the comparatively smalllateral expansion of a semiconductor laser of this kind, the electricalheat losses could have a negative impact on beam quality and stability.

It is further known to increase optical output power by providing,instead of a laser resonator integrated into the semiconductor body, aresonator equipped with an external resonator mirror. Such devices arealso known as VECSELs (Vertical External Cavity Surface EmittingLasers). Such semiconductor lasers having an external resonator usuallyexhibit far greater lateral expansion than a VCSEL and are operated atcorrespondingly higher powers. Thus, the diameter of a VECSEL istypically in the range of 10 μm or above. Due to the large difference inlateral dimensioning, designs for the conduction of operating current ina VCSEL usually cannot be transposed to a VECSEL.

SUMMARY

In certain embodiments, it is an object of the present invention tocreate a semiconductor body for a vertically emitting laser with highoutput power and improved current conduction. The semiconductor body isfurther intended to be producible with very little technicalexpenditure. In addition, in certain embodiments, it is an object of thepresent invention to specify a corresponding vertically emitting laserand a production method for an inventive semiconductor body.

In one aspect, a radiation-emitting semiconductor body with a verticalemission direction is disclosed that includes a current-conducting layerand a radiation-generating active layer, wherein the current-conductinglayer includes a current-blocking region and a current-permeable region.The semiconductor body can be provided with an external resonator toform a vertically emitting laser in which the external resonator has adefined resonator volume that overlaps with the current-permeable regionin the current-conducting layer of the semiconductor body.

Due to the adaptation of the current-conducting layer to the resonatorvolume defined by the external resonator, the operating current isconducted to a region suitable for generating radiation within theresonator volume. This makes it possible for the operating current to becoupled in through an electrical contact disposed outside the resonatorvolume, so that, on the one hand, this contact does not hinder theoutcoupling of radiation, and, on the other hand, the current conductionis certain to be advantageous for efficient laser operation.

Accordingly, in a preferred embodiment, the semiconductor body isprovided with a radiation output face and, on said radiation outputface, a defined radiation output region, an electrical contact forimpressing the operating current being disposed in the semiconductorbody outside this radiation output region. The current-blocking regionof the current-conducting layer is preferably disposed after theelectrical contact in the vertical direction, thereby preventing anyflow of current outside the resonator volume of the external resonatorthat would contribute inefficiently or not at all to the production ofradiation.

In an advantageous improvement, the current-blocking region is formed byat least one pn junction which in operation is a blocking pn junction.Such blocking pn junctions can be produced with comparatively littletechnical expenditure.

To this end, the current-conducting layer is preferably disposed betweentwo cladding layers of a first conduction type and is of a secondconduction type in the current-blocking region. In this arrangement, thecurrent-conducting layer forms a pn junction in the current-blockingregion in combination with each of the adjacent cladding layers, theforward directions of these pn junctions being mutually opposite andthus preventing current flow through the current-blocking region.Conversely, in the current-permeable region the current-conducting layeris of the first conduction type, and thus no blocking pn junctions aredisposed in that region.

The current-conducting layer is preferably doped in thecurrent-permeable region with dopants of the first and second conductiontypes. This facilitates the production of a corresponding semiconductorbody, since the current-conducting layer can first be doped throughoutwith dopants of the second conduction type. The current-conducting layeris subsequently doped specifically in the current-permeable region witha dopant of the first conduction type in such fashion that the doping ofthe first conduction type predominates and the conduction type isreversed, so that the current-permeable region as a whole is of thefirst conduction type.

In an advantageous refinement, a mirror structure designed to constituteone end of the external resonator is additionally disposed in thesemiconductor body. The mirror structure can be implemented for exampleas a Bragg mirror. This simplifies the construction of the externalresonator, because only one external mirror is necessary in addition tothe semiconductor body.

The external resonator mirror is preferably a concavely curved mirrorthrough which, further preferably, the laser radiation is coupled out.If the radius of curvature is suitably sized to the length of theresonator, such a resonator displays an advantageously higher stabilitycompared to a Fabry-Perot resonator with planar resonator mirrors.

Further provided in the present disclosure is a vertically emittingsemiconductor laser with an external resonator that contains asemiconductor body with a vertical emission direction.

To produce the semiconductor body, according to one or more embodimentsof the present invention, the semiconductor body is fabricated by anepitaxy process in which a first cladding layer of a first conductiontype, the current-conducting layer of a second conduction type and asecond cladding layer of the first conduction type are sequentiallygrown, and the current-conducting layer is subsequently doped with adopant within the current-permeable region in such a way that thecurrent-permeable region is of the first conduction type.

A production method of this kind requires comparatively little technicalexpenditure, since only structured diffusion within thecurrent-permeable region is necessary in addition to the epitaxialgrowth of the semiconductor layers.

Preferably, in order to dope the current-permeable region, a dopantsource comprising the dopant concerned is applied in a structured mannerto the second cladding layer in a region that is disposed after thecurrent-permeable region in the vertical direction, and the dopant issubsequently diffused into the current-permeable region. After thediffusion process, the dopant source can be removed.

Additional features, advantages and applications of the invention willemerge from the following description of two exemplary embodiments inconjunction with FIGS. 1 and 2.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view of an exemplary embodiment of aninventive semiconductor body or inventive semiconductor laser, and

FIGS. 2 a, 2 b, and 2 c is a schematic depiction of an exemplaryembodiment of an inventive production method in the light of threeintermediate steps.

Like or like-acting elements are provided with the same referencecharacters in the figures.

DETAILED DESCRIPTION

The exemplary embodiment depicted in FIG. 1 comprises a semiconductorbody 1 that includes, viewed in the vertical direction, a substrate 2, amirror layer 3, an active layer 4, a first cladding layer 5, acurrent-conducting layer 6 and a second cladding layer 7. Formed on theradiation-decoupling face 8 of the semiconductor body is a firstelectrical contact 9, preferably in the form of a ring contact.Correspondingly, a continuous electrical contact 10 is disposed on theopposite face of the semiconductor body.

The laser resonator is formed by the mirror layer 3 and an externalmirror 11, preferably with a curved mirror surface. Mirror layer 3 canbe implemented in a manner known per se as a Bragg mirror with aplurality of alternating semiconductor layers having different indexesof refraction. A suitable material system for such layers is, forexample, GaAs/AlGaAs, the layer sequence being composed of layer pairs 3a, 3 b of different aluminum concentrations. Alternatively, mirrorstructure 3 can also be a metal mirror or a dielectric mirror, in whichcase substrate 2 can advantageously be omitted. Moreover, in the case ofa metal mirror, this mirror can simultaneously be used as contact 10.

Current-conducting layer 6 is disposed between two cladding layers 5 and7 of a first conductor type and is also of the first conductor type incurrent-permeable region 13. In the current-blocking regions 12, on theother hand, current-conducting layer 6 is of a second conductor type, sothat, viewed in the vertical direction, two pn junctions that preventvertical current flow in current-blocking region 12 are formed with theadjacent cladding layers 5 and 7.

Substrate 2 can be provided for example as a n-doped GaAs substrate onwhich a n-doped GaAs/AlGaAs Bragg mirror is grown.

Active layer 4 is preferably fashioned as a single quantum well (SQW) ormultiple quantum well (MQW) structure. Such quantum well structuresordinarily comprise one (SQW) or more (MQW) quantum well layers arrangedbetween barrier layers. For example, the quantum well structures cancontain InGaP, InGaAs or GaAs quantum well layers and/or AlInGaP orAlGaAs barrier layers. Spacer layers can further be provided between theindividual quantum wells.

The term “quantum well structure” in the context of the applicationencompasses any structure in which charge carriers undergo quantizationof their energy states by confinement. In particular, the term “quantumwell structure” carries no implication as to the dimensionality of thequantization. It therefore includes, among other things, quantum wells,quantum wires and quantum dots and any combination of these structures.

Further suitable as active layer 4 is, for example, a p-doped InGaPlayer embedded between two AlGaInP layers.

This is followed, viewed in the vertical direction, by a p-doped firstcladding layer 5, a current-conducting layer 6 that is n-doped in thecurrent-blocking regions 12 and p-doped in the current permeable region13, and a p-doped second cladding layer 7. These three semiconductorlayers 5, 6 and 7 can contain for example AlGaAs or AlGaInP. A blockingnpn junction, i.e. two opposite pn junctions connected in series, isthereby formed in the current-blocking regions 12.

The lateral width of the current-permeable region 13 is so selectedaccording to the invention that the current-permeable region overlapswith the defined resonator volume 14 of the external resonator, thecurrent-permeable region preferably being wider than the lateral crosssection of the resonator volume 14 in the region of current-conductinglayer 6. In the exemplary embodiment shown, the resonator volume isdetermined in particular by the resonator length in combination with theradius of curvature of the external mirror 11. In case of doubt, theboundary of the resonator in the present invention can be taken in agaussian approximation as the 1/e² radius of the field of thefundamental electromagnetic mode.

FIGS. 2 a, 2 b and 2 c illustrate a production method for thesemiconductor body shown in FIG. 1 in the light of three intermediatesteps.

In a first step (FIG. 2 a), grown epitaxially on a substrate 2 is,first, a mirror structure 3 comprising a sequence of layer pairs 3 a, 3b having different indexes of refraction, an active layer 4, a firstcladding layer 5, a current-conducting layer 6 and a second claddinglayer 7. First cladding layer 5 is of a first conduction typethroughout, current-conducting layer 6 is of a second conduction typethroughout, and second cladding layer 7 is of the first conduction typethroughout. For example, first cladding layer 5 can be p-doped,current-conducting layer 6 n-doped and second cladding layer 7 againp-doped.

In a second step (FIG. 2 b), the current-blocking region 12 and thecurrent-permeable region 13 of current-conducting layer 6 are formed. Tothis end, a dopant source 15 is applied in a structured manner to thesecond cladding layer 7 in a region that is disposed aftercurrent-permeable region 13 in the vertical direction. The dopant source15 contains a dopant of the first conduction type, which is subsequentlydiffused into the layers thereunder and particularly intocurrent-conducting layer 6. The doping takes place in such a way thatthe conduction type is reversed in current-permeable region 13 ofcurrent-conducting layer 6. A vertical current path of the firstconduction type is thereby formed in the current-permeable region. Asuitable dopant for the above-cited semiconductor materials is zinc, forexample.

In a third step (FIG. 2 c), an electrical contact 9, for example a ringcontact, is subsequently formed on the radiation-decoupling face 8 ofthe semiconductor body, and a continuous electrical contact 10corresponding thereto is formed on the opposite face of thesemiconductor body. The ring contact is in the process arranged so thatthe ring opening is disposed after the current-permeable region 13 inthe vertical direction. This ensures that contact 9 will not encroach onthe provided resonator volume and adversely affect the decoupling ofradiation from the semiconductor body.

The explanation of the invention with reference to the exemplaryembodiments should not be construed as limiting the invention thereto.In particular, the present invention is not limited to the citedsemiconductor materials, and the layers of the semiconductor body cantherefore also contain another material, such as, for example,

In_(x)Al_(y)Ga_(1-x-y)As wherein 0≦x≦1, 0≦y≦1 and 0≦x+y≦1,

In_(x)Al_(y)Ga_(1-x-y)P wherein 0≦x≦1, 0≦y≦1 and 0≦x+y≦1,

In_(x)Al_(y)Ga_(1-x-y)N wherein 0≦x≦1, 0≦y≦1 and 0≦x+y≦1,

In_(x)Al_(y)Ga_(1-x-y)As_(u)N_(1-u) wherein 0≦x≦1, 0≦y≦1, 0≦x+y≦1 and0≦u≦1,

In_(x)Al_(y)Ga_(1-x-y)As_(u)P_(1-u) wherein 0≦x≦1, 0≦y≦1, 0≦x+y≦1 and0≦u≦1, and/or

In_(x)Al_(y)Ga_(1-x-y)P_(u)N_(1-u) wherein 0≦x≦1, 0≦y≦1, 0≦x+y≦1 and0≦u≦1.

The present invention further encompasses all combinations of thefeatures recited in the exemplary embodiments and the rest of thedescription, even if these combinations are not the subject matter of aclaim.

1. A method of producing a radiation-emitting semiconductor body with avertical emission direction, the method comprising: forming asemiconductor body including an active layer and a current-conductinglayer positioned between a first cladding layer and a second claddinglayer, a. the first cladding layer having a first conduction type, b.the current-conducting layer having a current blocking region of asecond conduction type and a current-permeable region of the firstconduction type; and c. the second cladding layer having the firstconduction type; and forming a laser resonator containing at least aportion of the semiconductor body to produce the radiation-emittingsemiconductor body forming the current-conducting layer having a secondconduction type throughout the current-conducting layer, thecurrent-conducting layer being formed over the first cladding layer inthe vertical emission direction; forming the second cladding layerhaving the first conduction type throughout the second cladding layer,the second cladding layer being formed over the current-conducting layerin the vertical emission direction; applying a dopant source including adopant of the first conduction type to the second cladding layer anddiffusing the dopant into the current-conducting layer to form thecurrent-permeable region by reversing the conduction type within aportion of the current-conducting layer, forming a vertical current pathin the semiconductor body through the first cladding layer, thecurrent-permeable region of the current-conducting layer and the secondcladding layer.
 2. The method of claim 1, where the current-conductingregion is doped throughout with a dopant of the second conduction typeprior to applying the dopant of the first conduction type to form thecurrent-permeable region.
 3. The method of claim 1, where the firstcladding layer and the second cladding layer are formed as p-dopedmaterials and the current-conducting layer comprises a n-doped materialin the current-blocking region and a p-doped material in thecurrent-permeable region.
 4. The method of claim 3, where each of thefirst cladding layer, the current-conducting layer and the secondcladding layer comprise a material selected from the group consistingof: AlGaAs and AlGaInP.
 5. The method of claim 1, further comprisingforming a mirror structure within the semiconductor body, the activelayer comprising a quantum well structure positioned between the mirrorlayer and the portion of the semiconductor body comprising the firstcladding layer, the current-conducting layer and the second claddinglayer.
 6. The method of claim 5, where the mirror structure is formed asa Bragg mirror with a plurality of alternating semiconductor layershaving different indices of refraction.
 7. The method of claim 6, wherethe semiconductor body further comprises a substrate and the mirrorstructure is formed by epitaxially growing the plurality of alternatingsemiconductor layers on the substrate.
 8. The method of claim 7, furthercomprising growing the active layer, the first cladding layer, thecurrent-conducting layer and the second cladding layer between aradiation decoupling surface and the substrate.
 9. The method of claim6, where the mirror structure comprises a plurality of alternating GaAsand/AlGaAs layers.
 10. The method of claim 5, where the active layercontains at least one material selected from the group consisting of:InGaP, InGaAs, GaAs, AlInGaP and AlGaAs.
 11. The method of claim 5,where the active layer comprises a p-doped InGaP layer embedded betweentwo AlGaInP layers.
 12. The method of claim 1, further comprisingforming a mirror structure within the semiconductor body, the activelayer comprising a quantum well structure positioned between the mirrorlayer and the portion of the semiconductor body comprising the firstcladding layer, the current-conducting layer and the second claddinglayer.
 13. The method of claim 12, where the dopant source compriseszinc.
 14. The method of claim 1, where semiconductor body comprises aplurality of layers including the first cladding layer, thecurrent-conducting layer and the second cladding layer, each layerformed from a material selected from the group consisting of: a.In_(x)Al_(y)Ga_(1-x-y)As wherein 0≦x≦1, 0≦y≦1 and 0≦x+y≦1, b.In_(x)Al_(y)Ga_(1-x-y)P wherein 0≦x≦1, 0≦y≦1 and 0≦x+y≦1, c.In_(x)Al_(y)Ga_(1-x-y)N wherein 0≦x≦1, 0≦y≦1 and 0≦x+y≦1, d.In_(x)Al_(y)Ga_(1-x-y)As_(u)N_(1-u) wherein 0≦x≦1, 0≦y≦1, 0≦x+y≦1 and0≦u≦1, e. In_(x)Al_(y)Ga_(1-x-y)As_(u)P_(1-u) wherein 0≦x≦1, 0≦y≦1,0≦x+y≦1 and 0≦u≦1; and f. In_(x)Al_(y)Ga_(1-x-y)P_(u)N_(1-u) wherein0≦x≦1, 0≦y≦1, 0≦x+y≦1 and 0≦u≦1.
 15. A method for producing aradiation-emitting semiconductor body with a vertical emissiondirection, the method comprising: a. providing a growth substrate, b.epitaxially growing an active layer comprising a quantum well structureover the growth substrate, and c. forming a current-conducting layerwith a current-blocking region over the active layer, wherein formingthe current-conducting layer comprises: i. growing a first claddinglayer of a first conduction type, ii. growing the current-conductinglayer of a second conduction type, iii. growing a second cladding layerof the first conduction type, and d. doping the current-conducting layerwith a dopant to form a current-permeable region within thecurrent-conducting layer by increasing the dopant concentration within aportion of the current-conducting layer to cause the current-permeableregion to be of the first conduction type; the current permeable regionbeing configured to overlap with a defined resonator volume of anexternal resonator to form a vertical emitting laser.
 16. The method ofclaim 15, wherein the doping of the current-conducting layer includesapplying a dopant source comprising the dopant to the second claddinglayer in a region formed over the current-conducting layer in thevertical emission direction, the dopant being subsequently diffused intothe current-permeable region, to form the current-permeable regionwithin the current-conducting layer.
 17. The production method of claim16, where the dopant source is removed after the diffusion of the dopantinto the current-conducting layer.
 18. The production method of claim15, wherein the dopant is zinc.
 19. A method of producing aradiation-emitting semiconductor body with a vertical emissiondirection, the method comprising: a. forming a semiconductor body havinga plurality of layers and formed by one or more steps including i.epitaxially growing an active layer comprising a quantum well structureover a growth substrate, and ii. forming a current-conducting layer witha current-blocking region and a current-permeable region over the activelayer, wherein forming the current-conducting layer comprises: 1.growing a first cladding layer of a first conduction type formed overthe active layer in the vertical emission direction,
 2. growing thecurrent-conducting layer of a second conduction type formed over thefirst cladding layer in the vertical emission direction, and
 3. growinga second cladding layer of the first conduction type over thecurrent-conducting layer in the vertical emission direction; thesemiconductor body comprising a plurality of layers including the firstcladding layer, the current-conducting layer and the second claddinglayer formed from a material selected from the group consisting of: i.In_(x)Al_(y)Ga_(1-x-y)As wherein 0≦x≦1, 0≦y≦1 and 0≦x+y≦1, ii.In_(x)Al_(y)Ga_(1-x-y)P wherein 0≦x≦1, 0≦y≦1 and 0≦x+y≦1, iii.In_(x)Al_(y)Ga_(1-x-y)N wherein 0≦x≦1, 0≦y≦1 and 0≦x+y≦1, iv.In_(x)Al_(y)Ga_(1-x-y)As_(u)N_(1-u) wherein 0≦x≦1, 0≦y≦1, 0≦x+y≦1 and0≦u≦1, v. In_(x)Al_(y)Ga_(1-x-y)As_(u)P_(1-u) wherein 0≦x≦1, 0≦y≦1,0≦x+y≦1 and 0≦u≦1; and vi. In_(x)Al_(y)Ga_(1-x-y)P_(u)N_(1-u) wherein0≦x≦1, 0≦y≦1, 0≦x+y≦1 and 0≦u≦1; b. applying a dopant source including adopant of the first conduction type to the second cladding layer; c.diffusing the dopant into the current-conducting layer to form thecurrent-permeable region by reversing the conduction type within aportion of the current-conducting layer, to form a vertical current pathin the semiconductor body through the first cladding layer, thecurrent-permeable region of the current-conducting layer and the secondcladding layer; and d. forming a laser resonator containing at least thecurrent-permeable region of the current-conducting layer of thesemiconductor body and the quantum well structure to produce theradiation-emitting semiconductor body.